Nat Hazards DOI 10.1007/s11069-014-1401-3 ORIGINAL PAPER
Controlling coal fires using the three-phase foam and water mist techniques in the Anjialing Open Pit Mine, China Zhenlu Shao • Deming Wang • Yanming Wang • Xiaoxing Zhong • Xiaofei Tang • Xiangming Hu
Received: 9 July 2014 / Accepted: 22 August 2014 Ó Springer Science+Business Media Dordrecht 2014
Abstract Coal fires are a serious environment, health, and safety hazard throughout the world. They damage the environment, threaten the health of people living nearby, burn away non-renewable coal, and result in significant economic losses. In this paper, the characteristics of the ignition and propagation of coal fires are illustrated first. Semienclosed environments (loose zones and abandoned roadways) favor the ignition of coal fires. The ‘‘upper fire’’ is pointed out to be prevalent and difficult to be controlled. Furthermore, the advantages and disadvantages of several commonly used techniques for controlling coal fires are analyzed. The three-phase foam and water mist techniques are believed to be effective in controlling coal fires, especially the ‘‘upper fires’’ in loose zones and abandoned roadways, respectively. Then, the three-phase foam coal fire extinguishing system is improved, and the water mist coal fire extinguishing system is developed. Finally, these two techniques are applied to control coal fires in the Anjialing Open Pit Mine. The results show that the three-phase foam and water mist techniques control coal fires efficiently and ensure the safe production of the mine as well as the security of personnel and equipments. Most importantly, this study provides a valuable method for the control of other coal fires. Keywords Coal fires Hazard Environment Fire control Three-phase foam Water mist
Z. Shao D. Wang Y. Wang X. Zhong X. Tang School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, China D. Wang (&) Key Laboratory of Gas and Fire Control for Coal Mines of Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu, China e-mail:
[email protected] X. Hu Department of City and Environmental Science, Binzhou University, Binzhou, Shandong, China
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1 Introduction Coal fires are fires that occur in underground coal seams featured with large area, high temperature, and long duration. Burning coal fires are typically caused by mining operations, which usually expose coal to air and provide favorable conditions for spontaneous combustion (Zhang et al. 2004b, 2007; Stracher et al. 2006; Kuenzer et al. 2007b). Walker (1999) notes that uncontrolled coal fires have been reported in the USA, Canada, China, Australia, India, Indonesia, South Africa, England, Germany, Poland, Czech Republic, Russia, Ukraine, Turkey, and Thailand. Coal fires have also been reported from Columbia, Egypt, France, Portugal, and New Zealand (Stracher et al. 2005; Masalehdani et al. 2007a, b). Among these countries, China, India, the USA, and South Africa suffer relatively more serious coal fires (Kuenzer and Stracher 2012; Kuenzer et al. 2008; Kuenzer 2005; Stracher 2004; Stracher and Taylor 2004; Zhang et al. 2004a). Coal fires directly burn valuable nonrenewable coal and indirectly cause billions of tons of coal to become unavailable (Tan 2000). Additionally, they release toxic gases and substances that threaten the health of local inhabitants (Finkelman 2004; Finkelman and Stracher 2011; Yang et al. 2005). Volatile elements such as arsenic, fluorine, and mercury are commonly enriched in coal and surrounding rocks. As the coal burns, these elements may volatilize and eventually be absorbed by food crops, condense on dust particles that are inhaled and ingested by humans and livestock, and bio-accumulated in birds, fish, and other animals (Keefer and Sajwan 1993). Toxic elements that concentrated in efflorescent minerals may be carried by rainwater and enter the hydrologic system, opening the possibility to other routes of exposure or ingestion by biota. Furthermore, the gases and heat released from the fires also lead to the death of vegetation (Rathore and Wright 1993). Coal fires also lead to the release of large amounts of greenhouse-relevant gases, such as CO2 and CH4 (Dai et al. 2002). In India, large numbers of people were displaced from their homes because of health problems associated with burning coal fires (Bharat Coking Coal Limited 2003; Stracher and Taylor 2004). The residents of Centralia were also relocated due to toxic gases and subsidence associated with coal fires nearby (DeKok 2000; Stracher et al. 2006). This hazard has become worse since the beginning of the industrial revolution (Stracher and Taylor 2004). Coal is a very important energy source of China and supports 65.7 % of the national primary energy consumption in 2013. Coal fires occur widely and seriously in China due to droughts, thick but shallowly buried coal seams, and excessive and indiscriminate mining. A maximum of 20 million tons of coal was estimated to be burned each year in China, which is equal to Germany’s annual hard coal production (Kuenzer 2007). Kuenzer et al. (2007a) calculated that coal fires in China alone account for *0.1 % of all global humaninduced CO2-equivalent greenhouse gas emissions. This was confirmed by the research of Van Dijk et al. (2011). Therefore, it is urgent to control existing coal fires, which will ensure the security of national energy, improve the ecological environment around fire zone, and protect the health of native residents. Controlling coal fires is a difficult, persistent, and costly problem throughout the world. Various techniques have been developed and applied to control coal fires (including coal mine fires) over the past decades. In general, methods for controlling coal fires include complete excavation of burning coal, surface sealing, water and grout injection, inorganic solidified foam, gel, liquid nitrogen (or carbon dioxide), and three-phase foam. Although these techniques have played important roles in controlling coal fires, they still have their own limitations (this will be discussed in detail in Sect. 3). Therefore, the control of coal fires is still a difficult problem.
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Fig. 1 Schematic cross-section of the occurrence and propagation of coal fires
In this study, the ignition and propagation of coal fires are first illustrated, from the point of fact we get that the ‘‘upper fire’’ in loose zone and abandoned roadway is prevalent in coal fire zone and is difficult to be controlled. Then, the advantages and disadvantages of several commonly used techniques for controlling coal fires are analyzed. The three-phase foam and water mist techniques are proved to be effective in controlling coal fires in loose zones and abandoned roadways, respectively, in theory. After introducing the characteristics of the three-phase foam and water mist, the corresponding coal fire extinguishing systems are designed and improved. Finally, these two techniques are applied to control coal fires in the Anjialing Open Pit Mine in 2013 by adopting the ‘‘zoning control’’ method. The results show that the temperature of the fire zone is cooled down within a short time. The threat of coal fires on the normal blasting and excavation of the mine is minimized, indicating that these two techniques can be used as an effective tool for the control of this kind of coal fires.
2 Ignition and propagation of coal fires Coal fire ignition can be forced or spontaneous. Forced ignition sources include lightning, brush, and forest fires as well as improperly controlled man-made fires (Kim 1977). In general, most coal fires are ignited by the spontaneous combustion of coal. The spontaneous combustion of coal is due to the heat produced by the exothermic reaction between coal and oxygen permeating through fissures and cracks (cf. Fig. 1). The convection of oxygen (air) is the foundation of spontaneous combustion and has two effects on the formation of coal fires. On the one hand, increased airflow enhances the oxygen supply to the reaction zone, leading to higher combustion velocity. On the other hand, heat transferred away from the combustion zone is also amplified, resulting in a substantial cooling effect. Fierro et al. (1999, 2001) investigated spontaneous combustion in stockpiles of Spanish black coal. They reasoned that the stock would not be in danger of spontaneous combustion if the air convection was either sufficiently low (=limited oxygen supply) or high (=cooling effect dominating). Therefore, coal fires usually occur in semi-enclosed environments (loose zones and abandoned roadways) that have been disturbed by a mining operation, which ensures a moderate oxygen supply and is also conducive to heat accumulation. A gradual accumulation of heat makes the temperature of coal increase
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gradually, which in turn promotes its oxidation (Zhu et al. 2013). When the temperature reaches the ignition point, the coal ignites. Once ignited, coal fires can burn for extended periods of time (decades of years) if they are left uncontrolled (Leitch 1940). As the coal is burned, a lot of exhaust hot gases (CO, CO2, SO2, etc.) are emitted to the surface during the combustion process (cf. Fig. 1). This column of hot gases is known as thermal ‘‘plume.’’ As the overburden is heated by the fire or as the coal pillars fail, the overburden subsides, creating new systems of fissures and cracks. Some of the fissures and cracks act as exhaust vents, while others act as air intakes to create self-sustaining conditions for the fire to propagate (Zeng 2012). Thermal buoyancy is upward force due to the molecular activity of the heated gases. Oxygen permeates into the burning center through fissures and cracks from the surface. The oxygen concentration declines from the surface to the burning center due to both thermal buoyancy and the consumption by the fire. In general, fires tend to propagate to the place where the oxygen concentration is high. Consequently, coal fire will propagate upward along the path of oxygen supply because of both the thermal buoyancy and the oxygen concentration resulting in ‘‘upper fire,’’ which is prevalent in coal fire zones and difficult to be controlled. The term ‘‘upper fire’’ indicates the upper part of burning coal fire. Correspondingly, the lower part of burning coal fire is called ‘‘lower fire,’’ which is much easier to be controlled. Most coal fires exhibit smoldering combustion, involving relatively small amounts of coal at any given time, with little visible flame. They can continue to burn in an atmosphere with oxygen concentration as low as 2 % (Scott 1944). Open pit mine is excavated on bench, when the excavation operation involves the fire zone, with abundant oxygen penetrating into these zones, the smoldering fires would turn into violence. As the bench moving forward, the fire zone will also spread along the bench. So, the propagation of coal fires in open pit mine is a dynamical process, fires mainly propagate along the excavation of benches.
3 Techniques for controlling coal fires Any fire requires three elements to propagate: fuel, oxygen, and a source of heat. If any one of the three sides of the fire triangle is removed, a fire cannot continue to burn (McPherson and Hinsley 1993). Coal fire control technique is based on the control or alteration of the three necessary fire elements. Fuel is removed when it is physically separated from the burning mass. Oxygen removal depends on either the introduction of an inert gas or the isolation of the fire zone from sources of fresh air. Heat removal can be accomplished by injecting a heat-absorbing material (usually water or inert gas). Completely excavating involves physically removing the overburden and burning coal, and cooling it to control the fire (Chaiken 1984). The hot material is cooled either by spraying it with water or by spreading it out on the ground and allowing it to cool in air. It can eradicate the fire effectively when the fire is located in a shallow area (20 m deep or less) and the burning area is small in scale. However, the fire may spread more rapidly due to the abundant oxygen supply after excavation, which will increase the risk and make the fire difficult to be controlled (Kim and Chaiken 1993). Surface sealing is a relatively inexpensive method of controlling coal fires (Shellenberger and Donner 1979; Kim and Chaiken 1993). It is intended to cut off the supply of the oxygen to the fire zone. If the seal can be maintained while all the stored heat dissipates, the fire may eventually be extinguished after a long time. During this period, the seal must be maintained. In general, some surface seals can be expected to fail due to many reasons such as subsidence, shrinkage, drying, rains, and increased fire activity.
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Water and grout injection is a conventional technique that has been used to control coal fires where enough water is available, and the water can be contained in the fire zone (Chaiken et al. 1984; Dalverny and Chaiken 1988, 1991). Water is used to lower the temperature of the burning material (heat removal). Water or grout can cover the burning material to stop the combustion reaction by oxygen exclusion (Bruhn and Michalski 1989). The water or grout level must cover not only all burning coal, but must also reach the overlying heated rock. However, due to the character of water flow, it cannot pile up to reach the upper fires and the overlying heated rock. When injecting water or grout through boreholes on the surface, gravity causes it to flow down dip, and erosion causes the size of the drainage channel to increase. Water may flow through channels and bypass the fire zone. The distribution of the water can only affects a relatively small volume around borehole, and most upper fires cannot be controlled. So, this method is limited to use on fires that are small and have been burning for a relatively short time to minimize the amount of stored heat. Inorganic solidified foam and gel are designed to fill the voids and fractures in an underground fire zone with fine, non-combustible solid material. The non-combustible material is intended to cover the burning material and fill the interstices in adjacent rock, limiting the amount of oxygen in the system and absorbing heat. However, these materials have problems to control coal fires with large areas due to their high cost and small diffusion range. Furthermore, the injection of large quantity of these materials will affect the quality of coal, which is ready to be mined after the fire is controlled. The storage and gasification equipment of liquid nitrogen and carbon dioxide are complex and costly (Chaiken et al. 1994). Gases are prone to diffusion with fissures and will not stay in the fire zone where they are needed. Therefore, it is difficult for these two techniques to control coal fires since fissures and cracks are widely distributed. According to our previous studies and field experience on coal fires, the vast majority of coal fires occur in loose zones and abandoned roadways that have been disturbed by mining operations (mainly mined by the room-and-pillar mining method). Coal fires rarely occur in undisturbed native coal seams. Furthermore, the fire tends to propagate upward because of both the thermal buoyancy and the oxygen concentration and eventually forms an upper fire. However, traditional fire controlling materials tend to flow to lower places because of gravity, which cannot be used to cover and control upper fires. In this case, even very small upper fire can serve as reignition point if the control measure fails to cover it and oxygen becomes available. Therefore, the key to successfully control coal fires is to ensure that the fire control materials cover and control the upper fires. The three-phase foam has good flow ability and has a wider coverage compared with water, slurry, and gel. More importantly, it can pile up quickly and take more non-combustible materials (water, and loess or fly ash) into the burning zone, which covers, cools, and controls the upper fire. This technique is especially applicable to fires in loose zones. However, the horizontal diffusion range of the foam far exceeded its stack height in abandoned roadway, which is characterized by large space. The foam flows away through the bottom of the roadway, leading to ineffective control of the upper fires in abandoned roadways. In recent years, the water mist technique, characterized by its non-pollution, low water consumption, low cost, and good fire suppression capability, has been used in fighting fires in libraries, civil architectures, transportation, and fuel storage areas. Although water mist cannot diffuse within loose zone, it can form three-dimensional coverage over the entire space in abandoned roadway, and in turn cover and control the upper fire. Therefore, these two techniques complement each other in controlling coal fires which mainly exist in loose zones and abandoned roadways.
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4 The three-phase foam and water mist techniques 4.1 Characteristics of the three-phase foam and water mist 4.1.1 Three-phase foam The three-phase foam is composed of a non-combustible solid material (loess or fly ash), inert gas (nitrogen), and water, which are all effective materials for fire control. The threephase foam increases approximately 20–30 times in volume after the nitrogen is injected into the slurry containing the foam agent. The water contained in the foam plays an important role in cooling and controlling fires. As the foam cells rupture, the nitrogen initially encapsulated in the foam is released and dilutes the concentration of oxygen in the fire zone. Furthermore, the loess or fly ash could cover the coal and minimize further oxidation. The three-phase foam has good flow ability and covers a wider area comparing with the water, slurry, and gel. More importantly, the foam could pile up quickly and takes more non-combustible materials (water, and loess or fly ash) to the burning zone, which would cover and cool the upper fires and heated roofs. 4.1.2 Water mist The term ‘‘water mist’’ refers to fine water sprays in which 99 % of the volume of the spray is in drops with diameters less than 1,000 microns (National Fire Protection Association 2010). The study and description of the fundamental principles of extinguishment of liquid and solid fuel fires by water mist can be traced back to the mid-1950s (Braidech et al. 1955). Fire suppression by water mist is mainly by a physical mechanism, and no significant chemical effects are involved. The mechanism of using a water mist system to extinguish fires is essentially different from traditional water injection methods. In addition to the cooling effect on the hot surface of combustible material, the water mist also extinguishes fires by smoke cooling, oxygen diluting, radiant heat attenuation, and a kinetic effect on the flame (Braidech et al. 1955; Liu et al. 2003; Mawhinney et al. 1994; Rasbash et al. 1960; Wighus and Aune 1995). The water mist is a promising method for controlling coal fires burning in abandoned roadways, which are characterized by large space and high temperature. Water droplets could disperse over the entire fire zone and reach the burning surfaces (especially the upper fires); therefore, the contact surface area between the water and high-temperature materials is enlarged and the utilization of water is improved. According to the characteristics of the three-phase foam and water mist, the authors believe that these two techniques can effectively control coal fires in loose zones and abandoned roadways, respectively. 4.2 Coal fire extinguishing system 4.2.1 Three-phase foam coal fire extinguishing system The three-phase foam coal fire extinguishing system consists of slurry device, nitrogen supply device, three-phase foam generator, and flow divider (cf. Fig. 2). The devices are connected by high-pressure hoses and fire hoses. The hoses are connected with the quick coupler. The system consists of water injection, grouting, and three-phase foam injection
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Fig. 2 Schematic diagram of the three-phase foam coal fire extinguishing system. This system is able to achieve water injection, grouting, and three-phase foam injection according to the specific circumstances of the fire
according to the specific circumstance of the fire and is characterized by strong pulping capability, large flow, and high mobility. During the fire control process, water from the primary water pipe could be used to directly control a fire. A branch pipe is drawn from the primary water pipe leading the water to the slurry tank; workers prepare the slurry by adding loess into the water. After stirring, the slurry is delivered to the grouting pipe. Part of the slurry could be delivered to the boreholes directly. The other slurry is delivered to a special pipe for the three-phase foam, where the foam agent is added to the slurry through a screw pump. The slurry and foam agent are mixed and then entered into the three-phase foam generator. A nitrogen source is connected to the three-phase foam generator to produce three-phase foam. A flow divider is designed to ensure the synchronous injection of several boreholes and improve the efficiency of fire control work. The foam is injected into boreholes after the flow divider at last. According to our previous fire control experience, when the three-phase foam is injected into boreholes located in loose zone, the foam easily overflows because it piles up easily (cf. Fig. 3a). Thus, we improved the system and developed a borehole-sealing device. Before injecting three-phase foam into borehole, the borehole-sealing device was utilized (cf. Fig. 3b). This prevented the foam from overflowing from borehole, thereby driving the foam to disperse horizontally and expand its coverage. 4.2.2 Water mist coal fire extinguishing system The fire in the abandoned roadway could be uncovered by two approaches: drilling and excavation. Each approach is used to form borehole and cavity that penetrating into the fire zone (cf. Fig. 4). The water mist coal fire extinguishing system is developed specially for these two types of approaches. It contains a twin-fluid water mist fire extinguishing device that atomized by a twin-fluid nozzle, and a single-fluid water mist fire extinguishing device that atomized by a centrifugal nozzle. The water mist generated by the twin-fluid water mist fire extinguishing device covers a larger space because it uses high-pressure water and high-pressure nitrogen as the power source. Thus, it is applied at the borehole, and the
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Fig. 3 Photographs showing the injection of the three-phase foam. a The three-phase foam erupted and overflowed from the borehole without the borehole-sealing device; b the borehole-sealing devices were installed in four high-temperature boreholes, and there was no foam overflow during the injection of the three-phase foam
Fig. 4 Schematic diagram of the water mist coal fire extinguishing system. This system contained a twinfluid water mist fire extinguishing device that atomized by a twin-fluid nozzle and a single-fluid water mist fire extinguishing device that atomized by a centrifugal nozzle; the system was applied at the borehole and the cavity of the burning abandoned roadway
single-fluid water mist fire extinguishing device is applied at the cavity, which is relatively open (cf. Fig. 4). In the twin-fluid water mist fire extinguishing device, water is pumped through the highpressure hose (No. 1) and the metal pipe in sequence, and it then arrives at the atomizing
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nozzle. In addition, the nitrogen is driven through a high-pressure hose (No. 2) and meets the water at the atomizing nozzle. Under the impact of the high-speed nitrogen, the water is separated into fine droplets and sprays out of the nozzle. The high-pressure water and nitrogen ensure a good atomization effect. Moreover, the bore diameter of the twin-fluid nozzle is relatively large and nozzle blockage is well avoided. In addition, the high-pressure nitrogen is effective in atomizing and also decreases the oxygen concentration in the space, improving diffusion of the fine droplets and increasing the diffusion range. High-pressure water is the only power source of the single-fluid water mist fire extinguishing device. Water is pumped through a high-pressure hose (No. 3). The water is in a rotational motion in the nozzle and separates into fine droplets under the centrifugal force. The centrifugal nozzle is installed on the bracket through the direction-adjusting knob, where the azimuth, angle, and height of spraying could be adjusted. The space at the cavity is relatively open; thus, the oxygen-diluting effect of the nitrogen is weak. The single-fluid water mist fire extinguishing device is only atomized by water instead of both water and nitrogen, which makes it much lighter and more flexible. The characteristics mentioned above show that the single-fluid water mist fire extinguishing device is more suitable to be settled before the cavity of the abandoned roadway.
5 Field work and results 5.1 Overview of coal fires in the Anjialing Open Pit Mine Shanxi is China’s major coal-producing province, whose identified coal reserves are approximately 266.4 billion tons and account for 22.6 % of the country’s total coal reserves. There are three coal bases in Shanxi, namely Jindong, Jinzhong, and Jinbei, respectively. Coal mines distribute in 91 of 118 counties in this province. In 2013, coal production of Shanxi was 960 million tons, accounts for 26 % of the total national production (3.692 billion tons). The Anjialing Open Pit Mine, locating in the Pinglu District of Shuozhou City in the Jinbei coal base, is one of the three open pit mines of the China Coal Pingshuo Group Co. Ltd. (the other two are the Antaibao Open Pit Mine and Donglutian Open Pit Mine, cf. Fig. 5). Its coal production was 32 million tons in 2013. The geologic units covering of this area consist of Carboniferous, Permian, Tertiary, and Quaternary. Ground surface is covered by Cenozoic loess. The coal-bearing stratum is the Taiyuan Formation of the Upper Carboniferous-Lower Permian, with an average thickness of approximately 90.37 m. There are 10 coal seams contained in the Taiyuan Formation, numbered 4(4-1), 4-2, 4-3, 5, 6, 7, 8, 9, 10, and 11 from top to bottom. The main recoverable coal seams are 4# (4-1), 4-2#, 9#, and 11#. The 8# coal seam is subordinate recoverable (cf. Table 1). The arsenic content of coal is between 3 and 5 lg/g. The coal seams are thick and shallow in this area, which has resulted in over-exploitation by many small coal mines in history. There are four abandoned coal mines in and around the Anjialing Open Pit Mine, i.e., Luzigou Coal Mine in the northern area, Houdong Coal Mine and Baixigou Coal Mine in the central area, and Anjialing Coal Mine in the southern area. The specific locations of these abandoned coal mines are illustrated in Fig. 6. All the abandoned mines adopted the room-and-pillar mining method. A relatively large proportion of coal was left in place, which was a serious risk of spontaneous combustion.
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Fig. 5 Location of the Anjialing Open Pit Mine within China and a satellite picture around it
In 2013, the excavation of 4# coal has entered the goaf of the abandoned Houdong Coal Mine, where the spontaneous combustion of coal is very severe. Many fires have been uncovered in this area (cf. Fig. 7). The boreholes for blasting could not be blasted because
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11
9
8
4-2
0–10.26
4(4-1)
4.1
2.06–8.55
14.53
10.75–16.8
1.21
0.8–1.85
6.1
1–8.85
5.71
Thickness (m) Minimum–maximum Average
Coal seam number
38.42
35.23
40.7
31.53
32.45
Ash content
3.28
2.65
3.62
1.35
0.52
Sulfur content
Gas coal
Gas coal
Gas coal
Gas coal
Flame coal
Coal type
Table 1 Characteristics of the main and subordinate recoverable coal seams in the Anjialing Open Pit Mine
5,016,000
15,808,900
–
8,217,600
8,482,000
Reserves in the Anjialing Open Pit Mine(t)
0.65
0.63
0.61
0.63
0.64
Rmax (%)
Nat Hazards
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Fig. 6 Abandoned coal mines in and around the Anjialing Open Pit Mine
Fig. 7 Photographs of the coal fires in the Anjialing Open Pit Mine. a Copious smoke emitted from boreholes, and the boreholes could not be blasted due to the high temperatures. b Flame ejected from the cavity of an abandoned roadway
the explosive could not be charged due to the high temperature, which seriously affected the excavation of the overlaying strata and the production of the 4# coal. Moreover, high temperatures and large amounts of toxic gases threatened the health of workers. To solve
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this severe problem, the three-phase foam and water mist techniques were applied to control the fire and sequentially ensured the safe production of the open pit mine. In open pit mine, the most important objective of fire controlling work is to quickly cool down the temperature of blast boreholes, and in turn ensure the safe blasting and excavation. As mentioned in Sect. 2, coal fires in open pit mines mainly dynamically propagate along the excavation of benches. So, fire control works mainly concentrate upon these areas. According to the temperature in blast boreholes and the type of fires (fires in loose zone or abandoned roadway), the three-phase foam or water mist technique will be adopted correspondingly. A ‘‘zoning control’’ method is applied during fire control operation. It can be interpreted as: taking a single blasting zone as a single fire control zone. When the temperatures of all boreholes in a single blasting zone are cooled down and stable for 2 days below the safe temperature of blasting, the blasting operation should be conducted immediately. Then, fire control work will transfer to the next blasting zone. Specific firefighting works are described in the following sections by taking two typical fire control zones as examples. 5.2 Controlling coal fires in loose zones using three-phase foam 5.2.1 Boreholes for fire control Many blast boreholes were needed for the mining operation in the Anjialing Open Pit Mine. The spacing of blast boreholes was approximately 8 m and could be used for the fire control operation. According to previous study (Shi 2010), when injecting three-phase foam into the boreholes (with a depth of approximately 15 m) for approximately 3 h with foam flow of 600 m3/h and outlet pressure of 0.7 MPa, the diffusion radius could reach 8 m. After a blast borehole was drilled, the sheathed thermocouple was immediately applied to measure the temperature in the borehole to detect the exact status of fire. In addition, boreholes must be blocked after the measurement to prevent air seepage into the burning zone, which may exacerbate the fire. 5.2.2 Injection of the three-phase foam Water was first injected into the borehole to cool down the lower fires. When the temperature no longer declined (or declined very slowly), the water injection was stopped and replaced by the three-phase foam injection. The purpose of injecting water first was to cool the lower fires with the lowest costs, since water is the cheapest firefighting material. However, water tended to flow downward because of gravity and did not stay in semi-enclosed loose zones. Only a relatively small area of upper fires around borehole could be cooled by water. Thus, most of the upper fires were uncontrolled, and the temperature in the boreholes remained unchanged during the later stage of water injection. In this case, the cooling effect by the water injection was very weak, and it did not make much sense to continue injecting water. So, the injection of the three-phase foam was adopted. The primary parameters used to prepare the high-quality three-phase foam are shown in Table 2. The ratio of water to loess fluctuated from the benchmark (4:1); at first, the foam had larger amounts of water and toward the end had more loess. Three to five boreholes were controlled in each injection. The borehole-sealing devices were used to seal boreholes to prevent the foam from overflowing and expand its coverage (cf. Fig. 3b).
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Ratio of water to loess (volume ratio)
Mixed slurry (m3/h)
Production of the three-phase foam (m3/h)
Ratio of foaming agent to slurry (mass percent)
30
4:1
20
About 600
0.5
Table 3 The composition and concentration of some gas samples in the Anjialing Open Pit Mine Place
CO (%)
CO2 (%)
SO2 (%)
CH4 (%)
C2H4 (%)
C2H6 (%)
C2H2 (%)
C3H8 (%)
No. 3 borehole of the first fire control area
12.87
33.22
0.04
6.19
0.03
0.43
0.04
0.14
Borehole in the third abandoned roadway
7.41
32.23
0.02
4.36
0.03
0.42
0.02
0.13
Cavity in the third abandoned roadway
0.18
0.25
0.01
0.32
0.00
0.01
0.02
0.01
5.2.3 Analysis of the fire control results Taking the No. 3 borehole of the first fire control zone as example, the initial temperature was as high as 374 °C. A lot of smoke with a strong pungent odor emitted from the borehole. The CO concentration was as high as 12.87 %. The SO2 and C2H2 were also detected, with the concentration of both 0.04 % (cf. Table 3). The water injection was conducted on June 3. The temperature decreased quickly to 286 °C in the first day (cf. Fig. 8). However, in the following 2 days, although the water injection was still carried on, the temperature dropped much more slowly. This indicated that the lower fires were controlled successfully by water injection, but water failed to cover and control upper fires. Since the cooling effect by water injection was no longer significant, the injection of the three-phase foam was adopted on June 6. The temperature decreased from 261 to 113 °C after injecting three-phase foam for just 1 day. After the temperature was below 100 °C, it dropped slowly. It was because that the evaporation of water significantly decreased below 100 °C. The heat taken away by water vapor also reduced dramatically. This phenomenon is common in fire control process by using whatever techniques. We stopped the injection on June 14, when the temperature dropped to 36 °C. In the next 2 days, the temperature kept stable and showed a declining trend, which met the prerequisite for borehole blasting operation. The other high-temperature boreholes were controlled by the same process. After the temperatures of all boreholes dropped below the safe temperature of blasting, the first fire control zone was blasted and excavated safely. Then, the fire control equipments were moved to the next fire control zone and that cycle repeated. 5.3 Controlling coal fires in abandoned roadways using water mist 5.3.1 Application of the water mist coal fire extinguishing system The twin-fluid and single-fluid water mist fire extinguishing devices were applied in the borehole and cavity to control fires in abandoned roadways in the Anjialing Open Pit Mine. The scheme consisted of three stages:
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Fig. 9 Scene photographs of the nitrogen–water twin-fluid fire extinguishing device (a) and the single-fluid water mist fire extinguishing device (b)
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Fig. 10 Thermal infrared figures of the borehole before (a) and after (b) spraying water mist
1. 2.
3.
Before spraying the water mist, an infrared thermal imager was used to measure the temperature in the borehole and cavity. The water mist coal fire extinguishing system was installed according to Fig. 4. After the installation, the air pressure and water pressure were adjusted to ensure that the system was in the best working condition. Then, the two devices were placed in the borehole and cavity (cf. Fig. 9). The spray orientation, angle, and height were adjusted according to the actual location of the fire. During the fire control process, the infrared thermal imager was used to measure the temperature distribution in the borehole and cavity every 3 h. The temperature was measured half an hour after the spraying was stopped.
5.3.2 Analysis of the fire control results Taking coal fires in the third abandoned roadway as example, the fire was uncovered by several boreholes and a cavity, where the twin-fluid and single-fluid water mist fire extinguishing devices were separately applied. The temperature in the borehole was very high (216.7 °C) before spraying with water mist (cf. Fig. 10a). The smoke emitted from the borehole contained CO (7.41 %), CO2 (32.23 %), SO2 (0.02 %), CH4 (4.36 %), C2H4 (0.03 %), C2H6 (0.42 %), C2H2 (0.02 %), and C3H8 (0.13 %) (cf. Table 3). During the fire control process, the smoke was significantly suppressed, and a large amount of water vapor emitted from the borehole nearby. A significant amount of heat in the fire zone was absorbed and taken away by water vapor, and the temperature dropped quickly. The maximum temperature dropped to 37.4 °C after approximately 40 h of fire-fighting operation (cf. Fig. 10b). In the following 2 days, the temperature was still similar to the surface temperature, indicating that the fire was controlled successfully. Before spraying water mist, a temperature up to 240.3 °C was detected in the cavity (cf. Fig. 11a). The concentrations of CO and SO2 were lower than that in borehole (cf. Table 3). It was because that the cavity was more open than borehole. The concentrations of CO, SO2, and other hydrocarbon gas were diluted by the fresh air around cavity. Plenty of water vapor also emitted from the cavity during the spraying. This indicated that the water mist was cooling down the burning coal and heated rocks. The maximum
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Fig. 11 Thermal infrared figures of the cavity before (a) and after (b) spraying water mist
temperature in the cavity dropped to 65.9 °C (cf. Fig. 11b). Afterward, little water vapor was emitted from the cavity. The temperature was monitored in the following 2 days and finally dropped to 45.3 °C, indicating that the fire here was also controlled and met the prerequisite for borehole blasting operation. After that, this zone was also blasted and excavated safely. In the year 2013, approximately 200,000 m2 of coal fires in the Anjialing Open Pit Mine were controlled by adopting the three-phase foam and water mist techniques. The original burning zones were blasted successfully after being controlled. No personal injury or mechanical loss that caused by coal fires occurred. These two techniques ensured the safe production of the Anjialing Open Pit Mine and the security of personnel and equipments.
6 Conclusions This paper analyzed the ignition and propagation of coal fires as well as the advantages and disadvantages of several commonly used techniques for controlling coal fires. A coal fire control method using the three-phase foam and water mist techniques was proposed, and corresponding coal fire extinguishing systems were designed and improved. Finally, these two techniques were applied to control coal fires in the Anjialing Open Pit Mine in 2013. The following conclusions were obtained from this study: 1.
2.
The vast majority of coal fires occur in semi-enclosed loose zones and abandoned roadways that have been disturbed by mining operations. Coal fires tend to propagate upward along the path of oxygen supply due to both the thermal buoyancy and the oxygen concentration, resulting in upper fires, which are prevalent in fire zones and difficult to be controlled. Most coal fires exhibit smoldering combustion with little visible flame. In open pit mines, the propagation of coal fires is a dynamical process along the excavation of the benches. The key to successfully control coal fires is to ensure that the fire extinguishing materials could cover and control the upper fires. However, commonly used coal fire control techniques, such as complete excavation, surface sealing, water and grout
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3.
4.
5.
injection, inorganic solidified foam, gel, and liquid nitrogen (or carbon dioxide), have limitations to achieve this goal. The three-phase foam has good diffusion capability and can pile up in loose coal–rock zones. However, in abandoned roadway that is characterized by large space, the horizontal diffusion range of the foam far exceeds its stack height, which leads to ineffective control of upper fires. Conversely, although water mist cannot diffuse within loose zone, it can form a three-dimensional coverage over the entire space in abandoned roadway and, in turn, control the upper fires in it. These two techniques have their own advantages in controlling different types of coal fires and complement each other. The borehole-sealing device is specially developed for the three-phase foam coal fire extinguishing system to prevent the foam from overflowing from the borehole, which can expand the coverage of the foam. The water mist coal fire extinguishing system containing the nitrogen–water twin-fluid and single-fluid water mist fire extinguishing devices is developed. These two techniques were applied to control coal fires in the Anjialing Open Pit Mine in 2013 by adopting the ‘‘zoning control’’ method. The results showed that the threephase foam and water mist techniques efficiently controlled coal fires and ensured the safe production of the Anjialing Open Pit Mine as well as the security of personnel and equipments. More importantly, it provides a valuable method for the control of other coal fires.
Acknowledgments The project was sponsored by The Joint Funds of the National Natural Science Foundation of China and the Shenhua Group Corporation Limited (No. 51134020), the National Natural Science Foundation of China (No. 51106175), and the Research and Innovation Program for College Graduates of Jiangsu Province (No. KYLX_1412).
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